RO129798A2 - Resistive acetone vapour sensor - Google Patents

Abstract

The invention relates to a resistive acetone sensor meant to detect and measure the concentration of acetone vapour in the air. According to the invention, the sensor uses as a sensitive element a ceramic material with submicronic structure prepared from the oxide compound with the composition LaPbFeZnO, where x = 0.05...0.5.

Description

The invention relates to a resistive reducing gas sensor for detecting and measuring the concentration of acetone vapor in the air. The invention aims to solve the problem of making a simple, sensitive acetone vapor sensor, with good selectivity over other gases, cheap, stable and with good contamination resistance.

The sensor is a device that detects or measures certain conditions or properties and records, indicates or sometimes responds to the information received. Thus, sensors have the function of converting a stimulus into a measurable signal. The stimuli may be of mechanical, thermal, electromagnetic, acoustic or chemical origin while the measurable signal is typically electrical in nature, although pneumatic, hydraulic and optical signals can also be used.

The main characteristics of the sensors can be defined by the following parameters: range of use, sensitivity, resolution (lowest measurable stimulus pitch), maximum frequency of the detectable stimulus (speed), accuracy (measurement error reported in percentages, at full scale), sensor size and mass, operating temperature and environmental conditions, lifetime (in hours or number of operating cycles), long-term stability and cost.

A gas sensor is a converter that detects gas molecules and produces an electrical signal whose value is proportional to the gas concentration. The devices with gas sensors can be classified into three groups, each group differs depending on the technology applied for their development: optical systems, spectroscopic systems and solid state systems.

Optical systems measure the absorption spectrum after the target gas has been excited to light. Such a sensor requires a complex system: a monochromatic excitation source and an optical sensor for the absorption spectrum analysis.

Spectroscopic systems are based on direct analysis of the molecular mass or the vibrational spectrum of the target gas. Such a sensor can quantitatively measure the composition of different gases with very good accuracy. Mass spectrometer and gas chromatograph are the most important sensor systems for gas spectroscopy but at the same time they are very expensive and difficult to implement in small spaces.

Solid state sensor systems are based on changing the physical and chemical properties of sensitive materials when exposed to various gaseous environments. Due to their simplicity and low cost, the oxide semiconductor materials, massive materials, thick layers or thin layers, are suitable for the recalibration of the gas / rectification gas. Dr. SgfurVĂsile, / _l VA ^f 'f

02013-00165-2 1 4) 2- 2013 resistive type. One of the detection mechanisms in these materials is largely based on the reactions that occur on the surface of the sensor as a result of the change in the concentration of the adsorbed oxygen. Oxygen ions adsorbed on the surface of the material (considering a n-type semiconductor material) extract electrons from the material and create a potential barrier that limits the movement of electrons and conductivity. When reactive gases are combined with this oxygen, the height of the potential barrier is reduced and thus the conductivity increases. This change of conductivity is directly related to the quantity of specific gas present in the environment, hence the possibility of determining the presence and concentration of the gas. These gas sensor reactions occur mainly at high temperatures (150 - 600 ° C) so the sensor must be positioned at maximum response temperature. Most solid-state resistive sensors for acetone vapors have low sensitivity, stability and reproducibility of measurements, while having low selectivity compared to other gases, greatly falsifying the measurement results. An advantage is presented by the resistive sensors in solid state in the form of thin layers in that they have a low energy consumption but are sensitive to physical and chemical contamination being rarely renewable.

The resistive acetone vapor sensor according to the invention removes the disadvantages of the presented sensors in that, in order to obtain a high sensitivity, a wide range of measurement of the acetone vapor concentration, a good stability and a long operating life, it is realized below the form of a porous ceramic block of the oxide compound with the Lao ^ Pbo ^ Fei-xZnxOs composition where x = 0.05 ^ 0.5. The ceramic block is provided with two interdigital electrodes (comb-shaped) on one of the surfaces and the opposite surface is placed on a heater. The material from which the ceramic block is made is an oxide compound, with a perovskite type structure and with an electrical resistivity of approx. 90 Ω-cm at 330 ° C, temperature found experimentally to be the optimum operating temperature. The sensor has a high sensitivity in a wide range of acetone vapor concentrations in air (50 ppm - 2000 ppm), as well as good physical and chemical stability. The ceramic block is made by pressing a very fine powder, nanometric, followed by a heat treatment, so as to obtain a mechanically resistant mass, with submicron crystals and with a porosity of approx. 40%. The active surface, in contact with the air containing acetone vapors, is very large, being mostly inside the sensor, not only at the surface of the material between the electrodes, so it is largely protected from solid impurities. For the chemical contamination of the entire active surface a large amount of contaminants would be needed. Ifeonil pOa.te is thermally cleaned (regenerated) because it is very resistant to temperature. 'Ț. _t «· ..! .

- y, of. Dr. tșan Vasile ί

J

/

Γ 1 3 “Ο Ο 1 6 5 -2 1 '82 - 2013

The good sensitivity to acetone vapors (C ₃ H ₆ O) can be explained by the involvement of the hydroxyl group OH '. When the perovskite sensor material (Lao, 8Pbo, 2Fei. _X Zn _x C> 3, x = 0.05-4), 5) is exposed to these vapors, the OH 'group reacts with the chemisorbed oxygen on the surface of the sensor generating free electrons. which increase the electrical resistivity of the perovskite (p-type semiconductor). in the presence of acetone vapors the chemical reaction between the specified gas and the oxygen adsorbed on the surface of the sensor material takes place:

C3H6O + 8O ⁿ (ads) -> 3CO2ads + 3H2O _to ds + 8ne (1)

Electrons released from reaction (1) tend to annihilate the gaps. Therefore the resistivity of the material (the electrical resistance from the terminals of the two electrodes of the disk) increases according to the concentration of acetone vapor.

The following is an example of the application of the invention to the development of a resistive reducing gas sensor for detecting and measuring the concentration of acetone vapor in the air. The procedure is as follows:

- a nanocrystalline powder of material (oxide semiconductor compound) with the Lao.sPbo ^ Feo ^ ZnojOs composition is prepared by the process according to the patent RO 121300/2007 which describes the preparation of a ferrite powder.

- from the prepared powder, presses with a pressure of 5000 daN / cm, presses with a diameter of 10 mm and a thickness of 1.8 mm;

- the pressed discs are heat treated in air for 40 minutes at 900 ° C. The result is a structure containing 0.15- ^ 0.25 µm crystals with a porosity of approx. 40%;

- the flat surfaces are polished, cleaned and on one of these surfaces two interdigital electrodes of platinum, gold, palladium or even silver are deposited;

- a disc prepared as above is mounted with the surface without electrodes on a thermostatic electric heater ensuring thermal contact with the entire surface again electrodes of the disc, heater capable of heating the disc at 330 ° C ± 5 ° C. The surface of the heater is electrically isolated from the contact surface of the disc by means of a small thin sheet or by the use of a heater with a dielectric surface. The heater-disc system is mounted in support with electrical contacts for the thermostat heater and the disc electrodes. The electrical contacts must connect through electrical cables to the heater's power supply, respectively to the electrical resistance measuring device between the disc electrodes, which may be a DC current ohmmeter or a microcontroller device that integrates the electrical resistance measurement, the conversion into units. ^ .m ^ of the concentration of acetone vapor in the air in parts per million (ppm) and transient of data.

The described sensor has a semiconductor behavior of the type - O ^ stence in

Ο 1 3 - 0 Ο 1 6 5 - 2 1 -02- 2013 the presence of acetone vapors (reducing vapors), has a porosity of 40%, the measurement range of acetone vapor concentration in air between 50 ppm and 2000 ppm for temperature of the 330 ° C heater, the variation of the resistance more than 500 times in the above mentioned measurement range, the response time between 45 s and 120 s depending on the acetone vapor concentration and the near-logarithmic concentration-resistance characteristic.

The sensor according to the invention has the following advantages:

- it is simple: a solid body of a single material provided with two interdigital electrodes and placed on an electric heater;

- it is physically and chemically stable: ceramic semiconductor material, resistant to high temperatures, without compounds that could react with substances from the environment;

- is very sensitive to acetone vapors and slightly sensitive to other vapors or reducing gases;

- covers a wide range of acetone concentrations in air;

- being porous and with a very large active surface, it is resistant to physical or chemical contamination;

- in case of contamination it can be thermally regenerated;

- it is cheap and easy to produce.

Claims (1)

CLAIMS: 1. Resistant sensor for acetone vapor characterized in that, in order to obtain sensitivity, selectivity, response time, superior stability and resistance to contamination, it uses as a sensitive element a ceramic material with submicronic structure made from the oxide compound with the Lao composition, 8Pbo, 2 Fe]. _x Zn _x C> 3 where x = 0.05 = 0.5 prepared by any process.